1. Field of the Invention
The present invention relates to an optical image measuring apparatus configured to project light to a measurement object made of a light scattering medium in particular, measure the surface morphology or inner morphology of the measurement object by using the reflected light or transmitted light of the projected light, and form an image of the measured morphology.
2. Description of the Related Art
In recent years, attention has been given to the optical image measuring technology of forming an image of the surface or inside of a measurement object by using a laser light source or the like. This optical image measuring technology is not hazardous to human bodies unlike a conventional X-ray CT. Therefore, development of applications of this technology in the medical field has been expected in particular.
An example of a typical method of the optical image measuring technology is the low-coherence interferometry (may also be referred to as the optical coherence tomography or the like). This method employs the low coherence of a broadband light source having a broad spectral width, such as a super luminescent diode (SLD), and enables detection of reflected light from a measurement object or transmitted light therethrough at superior distance resolution on the order of μm (refer to Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999), for example).
FIG. 9 shows a basic configuration of a conventional optical image measuring apparatus based on a Michelson interferometer, as an example of an apparatus employing the low-coherence interferometry. An optical image measuring apparatus 1000 comprises a broadband light source 1001, a mirror 1002, a beam splitter 1003, and a photodetector 1004. A measurement object 1005 is made of a scattering medium. A light beam from the broadband light source 1001 is split by the beam splitter 1003 into two, i.e., a reference light R traveling to the mirror 1002 and a signal light S traveling to the measurement object 1005. The reference light R is a light reflected by the beam splitter 1003. The signal light S is a light transmitted through the beam splitter 1003.
Here, as shown in FIG. 9, the z-axis direction is defined as a traveling direction of the signal light S, and the x-y plane is defined as a plane orthogonal to the traveling direction of the signal light S. The mirror 1002 is movable in a direction indicated by a double-headed arrow in FIG. 9 (z-scanning direction).
The reference light R is subjected to Doppler frequency shift by z-scan when reflected by the mirror 1002. On the other hand, the signal light S is reflected by the surface and inner layers of the measurement object 1005 when projected to the measurement object 1005. Because the measurement object 1005 is made of a scattering medium, the reflected light of the signal light S is thought to have a diffusing wave front having random phases including multiple scatter. The signal light reflected by the measurement object 1005 and the reference light reflected by the mirror 1002 and subjected to the frequency shift are superimposed by the beam splitter 1003, thereby generating interference light.
In image measurement using the low-coherence interferometry, only a component of the signal light S interferes with the reference light R, that a difference in optical path length between the signal light S and the reference light R is within the coherence length (coherent distance) on the order of μm of the light source, and that has a phase correlation with the reference light R. That is, only a coherent signal light component of the signal light S selectively interferes with the reference light R. Based on this principle, by moving the position of the mirror 1002 by z-scanning and changing the optical path length of the reference light R, a light reflection profile of the inner layer of the measurement object 1005 is measured. Further, scan with the signal light S projected to the measurement object 1005 is performed in the x-y plane direction. By detecting the interference light with the photodetector 1004 while performing the scan in the z direction and x-y plane direction, and analyzing an electric signal (heterodyne signal) outputted as the result of the detection, a 2-dimensional tomographic image of the measurement object 1005 is acquired (refer to Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
When the intensities of the reference light R and the signal light S superimposed by the beam splitter 1003 are denoted by symbols Ir and Is, and the difference in frequency between the reference light R and the signal light S and the difference in phase therebetween are denoted by symbols fif and Δθ, a heterodyne signal as shown by the following formula is outputted from the photodetector (for example, refer to Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).
Formula (1)i(t)∝Ir+Is+2√{square root over (IrIs)}cos(2πfift+Δθ)  (1)
The third term of the right side of the formula (1) indicates an AC electric signal, and the frequency fif thereof is equal to the frequency of beat between the reference light R and the signal light S. The frequency fif of an AC component of the heterodyne signal is called a beat frequency or the like. Here, the AC component is equivalent to an interference component of the heterodyne signal whose intensity periodically changes with time. The first and second terms of the right side of the formula (1) indicate DC components of the heterodyne signal, and correspond to the signal intensity of a background light component of the interference light.
However, in order to acquire a 2-dimensional tomographic image by the conventional low-coherence interferometry, it is necessary to scan the measurement object 1005 with a light beam and thereby successively detect reflected light waves from respective sites of the measurement object 1005 in a depth direction (z direction) and a tomographic face direction (x-y plane direction). Therefore, it takes a long time to measure the measurement object 1005, and it is hard to shorten a measurement time in consideration of the measurement principle.
In view of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 10 shows a fundamental configuration of an example of such an apparatus. An optical image measuring apparatus 2000 shown in FIG. 10 comprises a xenon lamp (a light source) 2001, a mirror 2002, a beam splitter 2003, a 2-dimensional photo-sensor array 2004 serving as a photodetector, and lenses 2006 and 2007. A light beam emitted from the light source 2001 is converted into a parallel light flux by the lenses 2006 and 2007, and a beam diameter thereof is widened. Then, the parallel light flux is split into two, i.e., the reference light R and the signal light S by the beam splitter 2003. The reference light R is subjected to Doppler frequency shift by z-scan of the mirror 2002. On the other hand, the signal light S enters the measurement object 2005 over a broad range of the x-y plane because the beam diameter has been widened. Therefore, the signal light S becomes reflected light containing information on the surface and inside of the measurement object 2005 in the incident range. The reference light R and the signal light S are superimposed by the beam splitter 2003, and detected by elements such as pixels and photo sensors arranged in parallel on the 2-dimensional photo-sensor array 2004. Thus, it becomes possible to acquire a 2-dimensional tomographic image of the measurement object 2005 in real time without scanning with a light beam.
As such a non-scanning type optical image measuring apparatus, an apparatus described in K. P. Chan, M. Yamada, and H. Inaba, “Electronics Letters”, Vol. 30, 1753 (1994) has been known. The apparatus described in this document is configured to input a plurality of heterodyne signals outputted from a 2-dimensional photo-sensor array into a plurality of signal processing systems arranged in parallel and detect the amplitude and phase of each of the heterodyne signals.
However, in order to increase the spatial resolution of an image for this configuration, it is necessary to increase the number of the elements of the array, and moreover, it is necessary to prepare a signal processing system provided with the corresponding number of channels to that of the elements. Therefore, it is supposedly hard to practically use the apparatus in fields that require a high resolution image, such as a medical field and an industrial field.
Thus, the inventors have proposed a non-scanning type optical image measuring apparatus described below in Japanese Unexamined Patent Application Publication JP-A 2001-330558 (claims, paragraphs 0068 to 0084 of specification, and FIG. 1). This optical image measuring apparatus comprises a light source, an interference optical system, and a signal processor. The light source emits a light beam. The interference optical system splits the light beam emitted from the light source into two, i.e., a signal light passing through a subject arrangement position in which a subject is arranged and a reference light propagating on an optical path different from an optical path passing through the subject arrangement position, and superimposes the signal light having passed through the subject arrangement position and the reference light having propagated on the different optical path, thereby generating interference light. The interference optical system includes: a frequency shifter that shifts the frequency of the signal light and the frequency of the reference light relatively to each other; light cutoff devices that, for reception of the interference light, split the interference light into two, and periodically cut off the interference lights split into two, thereby generating two trains of interference light pulses with a phase difference of 90 degrees therebetween; and photo sensors that separately receive the two trains of interference light pulses, each of the photo sensors having a plurality of light receiving elements spatially arranged and separately detecting light receiving signals. The signal processor combines the plurality of light receiving signals obtained by the photo sensors, and generates signals corresponding to respective points of interest on a propagating path of the signal light, of the surface or inner layer of the subject arranged in the subject arrangement position.
This optical image measuring apparatus with the configuration to split the interference light generated from the reference light and the signal light into two and receive with the two photo sensors (i.e., 2-dimensional photo-sensor arrays) is configured to have the light cutoff devices positioned before the respective photo-sensor arrays and sample the interference lights. A phase difference of π/2 is set to sampling periods of the two split interference lights, whereby the intensities of the signal light and reference light composing a background light component of the interference light and phase quadrature components (i.e., sine component and cosine component) of the interference light are detected. Moreover, the intensity of the background light component contained in outputs from the photo-sensor arrays is subtracted from the outputs from the photo-sensor arrays, whereby two phase quadrature components of the interference light are calculated, and the amplitude of the interference light is obtained using the result of the calculation.
An available image sensor such as a charge-coupled device (CCD) camera has been widely used as the 2-dimensional photo-sensor array. However, a problem has been recognized conventionally that a currently available CCD camera cannot follow the beat frequency of a heterodyne signal on the order of several kHz to several MHz because of the low frequency response characteristic thereof. The feature of the optical image measuring apparatus described by the inventors in JP-A 2001-330558 is measurement performed by using the low frequency response characteristic based on the sufficient recognition of the above problem.
In the conventional optical image measuring apparatus as described above, about ten interference light pulses are received by the CCD and accumulated to form a single image. Application of this apparatus to ophthalmic measurement will cause a problem that, in a case where an eye moves because of eyeball movement, heartbeat or the like during detection of the about ten interference light pulses, the interference light is subjected to Doppler shift at the time of reflection on a fundus oculi and the frequency thereof is modulated, whereby the accuracy of a formed image is lowered.
This problem will arise not only in the medical field such as the ophthalmic field but also in various kinds of fields using an object that may move during measurement as a measurement target (for example, the biological field).
Further, there is a problem that about ten interference light pulses are necessary to form a single image as described above and control of open/close timing of the light cutoff device therefor is difficult.
Furthermore, there is a problem that it is necessary to turn on/off the light source in synchronization with the beat frequency of the heterodyne signal and control of the synchronization is difficult.
Besides, the conventional configuration has a problem that about ten interference light pulses are necessary to form a single image and it takes time to measure.